I’m new to WordPress so there may be an easier way to find it, but for me editing the post is a reliable way to see it. Logged in to admin panel and editing the post I can see in my address bar
http://www.mysite.com/wp-admin/post.php?post=51&action=edit
So in that instance that’s post 51. Finding the ID that way holds true whether it’s a Page or a Post.
